The present invention relates to an ion trap device which uses a three-dimensional quadrupole electric field to trap ions in. Such an ion trap device can be used in a mass spectrometer or in an ion separator.
In an ion trap device, as mentioned above, ions are trapped in a three-dimensional quadrupole electric field generated basically by combining an RF electric field and a DC electric field. One type of ion trap device is constructed by electrodes whose inner surfaces are shaped hyperboloid-of-revolution so that a rather large ion trapping space is created in the space surrounded by the electrodes. Another type of ion trap device is constructed by cylindrical and disc electrodes (Cylindrical Ion Trap) in which an ion trapping space is created around the center of the space surrounded by the electrodes. In these constructions, the electrodes are composed of a ring electrode, and two end cap electrodes placed at both ends of the ring electrodes, wherein the RF voltage is normally applied to the ring electrode. In either electrode construction, the mass to charge ratio (m/e) of an ion determines whether the ion is trapped in the trapping space in a stable manner, or whether its movement becomes unstable and it collides with the electrodes, or it is ejected from an opening of the electrodes. The theory of an ion trapping method is explained in, for example, R. E. March and R. J. Hughes, xe2x80x9cQuadrupole Storage Mass Spectrometryxe2x80x9d, John Wiley and Sons, 1989, pp. 31-110.
The ion stability condition in which an ion is trapped in a stable manner in the three-dimensional quadrupole electric field is shown by the region of 0 less than xcex2r less than 1 and 0 less than xcex2z less than 1 in FIG. 1. The parameter of the ordinate az and that of abscissa qz are represented by the following equation:                                                                         a                z                            =                              -                                                      8                    ⁢                    eU                                                                              mr                      0                      2                                        ⁢                                          Ω                      2                                                                                                                                                              q                z                            =                                                4                  ⁢                  e                  ⁢                                      xe2x80x83                                    ⁢                  V                                                                      mr                    0                    2                                    ⁢                                      Ω                    2                                                                                                          (        1        )            
where U is the magnitude of the DC voltage, V is the amplitude of the RF voltage, xcexa9 is the angular frequency of the RF voltage, r0 is the dimension of the ring electrode (precisely, its radius at the center), m is the mass of the ion, and e is the electrical charge of the ion.
In many of the recent mass spectrometers using an ion trap device (ion trap mass spectrometers), ions are trapped in such an operation mode that the DC voltage (U) is not applied but the RF voltage (V) is solely applied. In this case, the above-mentioned parameters are on the qz axis or az=0. As seen from FIG. 1, only ions whose mass to charge ratio (m/e) corresponds to the value of qz less than 0.908 can be trapped in the ion trapping space in a stable manner.
Ions are trapped in the ion trapping space as follows. In one method, an electron beam or the like is injected into the ion trapping space so that ions are created in the ion trapping space. In another method, ions are created outside and introduced in the ion trapping space. In any case, only appropriate ions are trapped there and gathered to the center of the ion trapping space with a cooling gas filled in the ion trapping space.
FIG. 2 illustrates an ion analyzing method using a mass selective instability mode (ibid. p.330). First, ions are introduced and cooled (I). Then the RF voltage V is gradually increased (II). As it increases, ions of smaller mass to charge ratios become unstable, and some of these ions are ejected from an opening formed in an electrode. The ejected ions are detected and the amount (or the strength of the detection signal) is plotted against the RF voltage, which produces a mass spectrum (mass analysis).
FIG. 3 shows an MS/MS analyzing method in which the RF voltage is changed in a rather complicated manner (ibid. p.371). Ions are generated in the stage (I), and unnecessary ions of lower mass to charge ratios are eliminated in the stage (II). At this time, ions to be analyzed (precursor ions) are trapped in the ion trapping space. Then the RF voltage is lowered in the stage (III) to obtain the mass spectrum of fragment ions. At this time, the secular frequency of the precursor ions left in the ion trapping space is calculated. In the following stage (IV), an auxiliary RF voltage having the same frequency as the secular frequency of precursor ions is applied to the end cap electrodes. The auxiliary RF voltage creates a dipole electric field in the ion trapping space so that the precursor ions are excited and fragment ions are generated. As the RF voltage is gradually increased in the stage (V), the mass spectrum of the fragment ions is obtained.
In the above example, the ion trapping electric field is changed to select precursor ions. In another method, a special waveform of auxiliary voltage is applied to the end cap electrodes and the precursor ions are selected at high resolution (for example, in the U.S. Pat. Nos. 4,761,545 and 5,134,826). In such a method, the RF voltage is usually maintained constant with high accuracy. If there is an error in the RF voltage, secular frequency of the ions deviates from the original value and high resolution cannot be achieved. If the movement of the ions is not adequately cooled within a reasonable period of time before they are separated, there arises a discrepancy of the secular frequency of the ions and high resolution cannot be achieved, either.
When the RF voltage is intended to be changed and the setting value of the RF voltage is instantly set at the controller side, the actual voltage applied to the ring electrode cannot follow the quick change and it takes some time until the actual voltage reaches the setting value. Especially when the RF voltage is abruptly set to a lower value, a large undershoot occurs in the actual RF voltage. This disturbs ion""s stability in the ion trapping space and some of the ions are lost, which deteriorates the sensitivity and reproducibility of the analysis.
In actual analyzers, a resonant circuit having a high Q value is formed including the capacitance between the electrodes and an external coil. Using such a resonant circuit, a high RF voltage for trapping ions is generated from a rather low RF driving voltage. When the RF voltage is stable, the loss in the resonant circuit due to its resistance and the power supplied to it from the RF driving circuit are balanced. When the setting value of the output voltage of the RF driving circuit is abruptly changed in this state, the balance is lost, and the actual RF voltage applied to the ring electrode approaches the setting value with the time constant proper to the resonant circuit. The time constant of the resonant circuit is about 100 xcexcsec, and it increases as the Q value is increased.
When the setting value of the RF voltage is changed, the output voltage of the RF driving circuit does not become constant. Thus a feedback control is performed so that the monitored value of the actual RF voltage is equal to the setting value. When, for example, the setting value of the RF voltage is abruptly decreased, the actual voltage on the ring electrode cannot follow the change, and monitored value remains unchanged at first. This renders a large discrepancy between the monitored value and the setting value, which brings the output voltage of the RF driving circuit to zero. Then the RF voltage of the ring electrode decreases with the time constant of the resonant circuit. When the actual RF voltage approaches the setting value, the output voltage of the RF driving circuit begins to increase. The actual RF voltage once passes the setting value to a lower value, and then it bounces back and surpasses the setting value. This causes the output voltage of the RF driving circuit to decrease. Thus an undershoot or ringing occurs in the actual RF voltage, and the movement of the ions in the ion trapping space is disturbed. It takes a long time before the actual RF voltage, as well as the output voltage of the RF driving circuit, becomes stable.
In one method of avoiding such a problem associated with a feedback control, the setting value of the RF voltage is changed not abruptly but on a moderate slope to a target value. If the slope is moderate enough, the output of an error amplifier which compares the monitored value of the actual RF voltage with the setting value does not become too large. This eliminates an undershoot in the actual RF voltage, and prevents disturbing and losing ions in the ion trapping space. When the output voltage of the RF driving circuit is made to zero, the actual RF voltage V(t) of the ring electrode approaches to zero exponentially with the time constant xcfx84 of the resonant circuit, as follows.                               V          ⁡                      (            t            )                          =                              V            ⁡                          (                              t                0                            )                                ⁢                      exp            ⁡                          (                              -                                                      t                    -                                          t                      0                                                        τ                                            )                                                          (        2        )            
According to the above equation (2), the changing rate of the RF voltage changes from xe2x88x92V1/xcfx84 to xe2x88x92V2/xcfx84 while the RF voltage is decreased from V1 to V2. If the RF voltage is to be changed on a constant slope, the slope should be inclined less than the smallest value of their absolute value, i.e., xe2x88x92V2/xcfx84 in order for the error amplifier not to overshoot. If, then, the slope is set at xe2x88x92V2/T (where Txe2x89xa7xcfx84), for example, the time needed for the RF voltage to change from V1 to V2 is                                                                         V                1                            -                              V                2                                                    V              2                                ⁢          T                =                              (                                                            V                  1                                                  V                  2                                            -              1                        )                    ⁢          T                                    (        3        )            
This means that the time needed to change the RF voltage increases as the value of V2 decreases. It is also necessary to change the slope of the RF voltage setting according to the target value V2 of the RF voltage. When the RF voltage reaches the target value V2, a cooling time is further needed, which elongates the analyzing time and decreases the throughput of the system.
Thus an object of the present invention is to shorten the time necessary to change the RF voltage, and improve the throughput of the system.
Addressing the above problems, the present invention takes the following measures. In an ion trap device using an RF electric field to trap ions, when the amplitude of the RF voltage for generating the RF electric field is changed from a first value to a second value, it is changed according to the exponential function of time. And the time constant of the exponential function is set equal to or longer than the time constant of the resonant circuit for generating the RF electric field.
Owing to the above measures, the time necessary to change the RF voltage is shortened, and an overshoot, undershoot, or ringing of an actual RF voltage on the electrode or electrodes of an ion trap is avoided when the RF voltage setting value is changed, so that movement of ions in the ion trap is not disturbed and the throughput of the ion trap device is improved.